Examining the Reproductive Success of Bull Kelp ( Nereocystis luetkeana ) in Climate Change Conditions

Abstract Climate change is affecting marine ecosystems in many ways including rising temperatures and ocean acidification. From 2014-2016, an extensive marine heatwave extended along the west coast of North America and had devastating effects on numerous species during this period, including bull kelp ( Nereocystis luetkeana ). Bull kelp is an important foundation species in coastal ecosystems that can be affected by marine heat waves and ocean acidification, however these impacts have not been investigated on sensitive early life stages. To determine the effects of changing temperatures and carbonate levels on Northern California’s bull kelp populations, we collected sporophylls from mature bull kelp individuals in Point Arena, CA. At the Bodega Marine Laboratory, we released spores from field-collected bull kelp, and cultured microscopic gametophytes in a common garden experiment with a fully factorial design crossing modern conditions (11.63±0.54°C and pH 7.93±0.26) with observed extreme climate conditions (15.56±0.83°C and 7.64±0.32pH). Our results found that both increased temperature and decreased pH influenced growth and egg production of bull kelp microscopic stages. Increased temperature generally resulted in decreased gametophyte survival and offspring production. In contrast, decreased pH had less of an effect, but generally resulted in increased gametophyte survival and offspring production. Additionally, we found that increased temperature significantly impacted reproductive timing by causing female gametophytes to produce offspring earlier than under ambient temperature conditions. Our findings inform better predictions of the impacts of climate change on coastal ecosystems as well as provide key insight into environmental dynamics regulating the bull kelp lifecycle.